Photoresponsive devices, such as photovoltaic cells, photodetectors, photoconductors, and the like depend on incident light, frequently within a narrow bandwidth, for efficient operation. These devices carry substantially transparent films, coatings, and layers thereon, e.g. conducting electrodes, conducting contacts, etc. thereon. Incident light may be transmitted through a transparent medium, reflected thereby, or absorbed thereby. Transmitted light is the incident light minus the sum of reflected and absorbed light. As used herein, the sum of reflected and absorbed light is referred to as parasitic absorption. While apparently transparent, the above mentioned films, coatings, and layers have parasitic optical absorption associated therewith. That is, these films, coatings, and layers absorb and reflect light that would otherwise reach the photoresponsive device. This reduces the efficiency of the underlying photoresponsive device.
Accordingly, it can be seen that there exists a need for an anti-reflection, light bandwidth widening coating, film, or layer, and a method of depositing such coatings, films, or layers on semiconductor devices which will not diminish the desirable photoresponsive properties of the semiconductor material of the semiconductor device. This need is especially great in large area photovoltaic cells, where a small decrement in output per unit area, multiplied over a large area, represents a large absolute loss of output due to parasitic light absorption.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor alloy materials, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n type photovoltaic devices which are, in operation, substantially equivalent to their crystalline counterparts. It is to be noted that the term "amorphous", as used herein, includes all materials or alloys which have long range disorder, although they may have short or intermediate range order or even contain, at times, crystalline inclusions.
It is now possible to prepare amorphous silicon alloys by glow discharge deposition or vacuum deposition techniques, said alloys possessing (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) high quality electronic properties. Such techniques are fully described in U.S. Pat. No. 4,226,898, entitled Amorphous Semiconductors Equivalent To Crystalline Semiconductors, issued to Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980; U.S. Pat. No. 4,217,374, to Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, also entitled Amorphous Semiconductors Equivalent To Crystalline Semiconductors; and U.S. patent application Ser. No. 423,424 of Stanford R. Ovshinsky, David D. Allred, Lee Walter, and Stephen J. Hudgens entitled Method of Making Amorphous Semiconductor Alloys and Devices Using Microwave Energy. As disclosed in these patents and applications, fluorine introduced into the amorphous silicon semiconductor layers operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498, issued Aug. 16, 1960. The multiple cell structures therein disclosed utilized p-n junction crystalline semiconductor devices. Essentially, the concept employed different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device (by definition) has two or more cells with the light directed serially through each cell. In the first cell a large band gap material absorbs only the short wavelength light, while in subsequent cells smaller band gap materials absorb the longer wavelengths of light which pass through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell, while the short circuit current thereof remains substantially constant.
Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing sytem. Such continuous processing systems are disclosed in U.S. Pat. No. 4,400,409 for A Method Of Making P-Doped Silicon Films and Devices Made Therefrom; Ser. No. 244,386, filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Material, now U.S. Pat. No. 4,410,558; and Ser. No. 306,146, filed Sept. 28, 1981 for Multiple Cell Deposition And Isolation System And Method, now U.S. Pat. No. 4,438,723, Ser. No. 359,825, filed Mar. 19, 1982 for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells, now U.S. Pat. No. 4,492,181; and Ser. No. 460,629 filed Jan. 14, 1983 for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells, now U.S. Pat. No. 4,485,125. As disclosed in the above patents, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. In making a photovoltaic device of p-i-n type configurations, the first chamber is dedicated for depositing a p-type semiconductor alloy, the second chamber is dedicated for depositing an intrinsic amorphous semiconductor alloy, and the third chamber is dedicated for depositing an n-type semiconductor alloy. The resulting photovoltaic device is referred to, by order of deposition, as a p-i-n device.
The layers of semiconductor material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form a photovoltaic device including one or more p-i-n cells, one or more n-i-p cells, a Schottky barrier, photodiodes, phototransistors, photoconductors, or the like. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained.
In many cases, it is desirable to increase the fraction of light incident on the device that passes through nonphotoresponsive layers to the photoresponsive regions of the device. This may be accomplished by reducing parasitic light absorption, i.e., reflection and absorption, and increasing the bandwidth of light transmitted through the nonphotoresponsive layers.
One attempt to reduce parasitic absorption is disclosed in U.S. Pat. No. 4,389,534 to Gerhard Winterling for Amorphous Silicon Solar Cell Having Improved Antireflection Coating. As therein described, a layer of polycrystalline material is interposed between the transparent conductive oxide layer and the amorphous silicon photoresponsive device. This layer is formed of polycrystalline silicon. The polycrystalline layer introduces additional, difficult processing steps into the fabrication of the device, and could convert underlying amorphous silicon to polycrystalline or micro crystalline form. It is desirable to reduce parasitic absorption without providing a layer of polycrystalline silicon between the amorphous silicon layers and the transparent conductive oxide. Accordingly, the instant invention fulfills a long felt need in the production of semiconductor devices in general, and has special significance in the production of amorphous photovoltaic devices.
These and other advantages of the instant invention will become apparent from the drawings, the detailed description of the invention and the claims which follow.